This invention relates generally to a carding process for forming a nonwoven fibrous structure including a blend of short fibers under 1/4 inch in length and longer reinforcing fibers. In particular this invention relates to a carding process employing a main carding drum having projections on its outer periphery for cooperating with projections on rotating worker and turner rolls to separate and blend the fibers.
Dry-formed nonwoven fibrous structures including short fibers less than 1/4 inch in length as the predominant fiber component (by weight) and a minor proportion of longer reinforcing fibers are becoming quite popular; especially for limited and single use absorbent applications. For example, in the low basis weight range of less than about 6 ounce/yard.sup.2, these nonwoven webs have been employed in the formation of diaper covers, industrial and household wipers, cosmetic pads, liners for underpads and impregnated wipes for personal hygiene usage. At higher basis weight levels the fibrous structures can be employed as an internal absorbent component of sanitary napkins and disposable diapers, or as a feed mat to be directed into a web forming device to form nonwoven webs in the above-mentioned low basis weight range. A representative process employing a feed mat of blended short and long fibers to form such a low basis weight web is disclosed in U.S. Pat. No. 3,862,472, issued to Norton et al, and assigned to Scott Paper Company. This patent is incorporated herein by reference.
In U.S. Patent Application Ser. No. 715,165, filed on Aug. 17, 1976, and assigned to Scott Paper Company, a carding apparatus and method is disclosed for forming a fibrous structure including short fibers less than 1/4 inch in length as the predominate fiber component by weight, and longer reinforcing fibers as the minor fiber component by weight. This apparatus and method are most suited for use in forming high basis weight fibrous mats that can be used, for example, as the feed mat to a web former, as disclosed in the above-discussed Norton et al patent. However, this apparatus and method can also be used to form low basis weight fibrous structures less than about 6 ounces/yard.sup.2.
When the apparatus disclosed in the 715,165 application is operated with the working section under atmospheric pressure conditions, and with the main drum rotating at a surface speed in excess of 12,500 feet/minute, undesirable counter air flow conditions begin to develop about the satellite rolls. This tends to cause premature launching of the short fibers from the satellite rolls; resulting in the formation of clumps that become part of the formed web structure. Although these clumps are undesirable in many fibrous structures, they are most undesirable in low basis weight structures less than about 6 ounces/yard.sup.2. The reason for this is that uniformity of appearance and physical properties is important for many products in which these low basis weight webs are used. The presence of clumps has an adverse effect on both of these desired attributes.
This invention is an improvement over the apparatus and method generically described and claimed in the 715,165 application, and can be employed in forming both low basis weight webs less than 6 ounces/yard.sup.2, and higher basis weight feed mats; generally greater than 12 ounces/yard.sup.2. In accordance with this invention the greatest advantages are achieved in minimizing the formation of fibrous clumps in low basis weight webs less than about 6 ounces/yard.sup.2.